Understanding Gears: Choosing Rings and Cogs- Mtbr.com

# Thread: Understanding Gears: Choosing Rings and Cogs

1. ## Understanding Gears: Choosing Rings and Cogs

There are many posts regarding gearing and the typical questions are “what size chainring should I get,” and “what’s the largest cog that I need?” There are many replies that all talk in terms of the number of teeth: “get a 28t for the front and a 42t for the rear.” This type of information is not accurate and it is misleading because it doesn’t consider tire size, air pressure or the current rider’s baseline data.

Understanding gearing requires that you understand GEAR INCHES or in other words, mechanical advantage. Creating a gear chart for yourself will provide a clear picture of your current gearing and where you can go from there.

The basic formula for gear inches follows:
Gear Inches = drive wheel diameter x (Front chainring teeth/Rear cog teeth)

There are many Gear Inch calculators on the Web although they are not accurate for your specific situation. These calculators use standard wheel diameters which are inaccurate because they don’t consider variations in tire or rim sizes or tire pressures. (This is why when you calibrate your bike computer, you should actually measure the circumference of your wheel.)

To calculate YOUR Gear Inches, you need to actually measure the circumference of your wheel. This will vary depending on rim width, tire size, and tire pressure. Therefore, use the following formula to calculate gear inches:

Gear Inches = (Circumference/π) x (Front chainring teeth/Rear cog teeth)

Circumference (“roll out”) measurement: 1) set your typical riding tire pressure, 2) lay out a tape measure on a flat surface, 3) put on your typical riding gear including pack, 4) move the rear tire’s valve stem to the bottom of the tire (6 o’clock position) and align it to the tape measure, 5) sit on bike and roll forward until the valve stem makes one complete revolution – note the distance. (It’s helpful to hold on to something/someone when measuring.)

Example: The circumference of my wheel measured 87” (4.6” Ground Control, 90mm rim, 7psi @ 45 degrees F). My lowest gear is 22T front and 36T rear (I have a 2x setup).

Gear Inches = (87 / 3.14) x (22/36)
= (27.7) x (.61)
= 16.9”

Example: If I were considering a 1x setup I would calculate my gear options so I could come close to my current low gear since I want to keep that low gear.

Gear Inches = (87 / 3.14) x (26/42)
= (27.7) x (.62)
= 17.17”

To see your complete gear chart, simply make a spreadsheet (below), put the teeth per ring in the left column and teeth per cog in the horizontal row. The blank spaces in the chart below put your Gear Inch formula in them so the data will automatically populate for you. Add all of the different ring and cog sizes you’re considering. This chart is especially helpful when going from a 2x to a 1x crank setup. A gear chart is also helpful to see how much difference there is between each gear so you can plan more even steps between gears:

 12 14 17 25 29 35 38 42 44 22 24 26 28 30 32

Notes:

• The numbers above are for illustration purposes. You will use your actual ring/cog teeth numbers you current have and those you are considering.
• You don’t need a gear chart for every tire pressure you run. The key to this exercise is to get you close to YOUR actual data in order to make an informed decision about your gears.

I hope you find this useful. Please feel free to add any information that would be helpful.

Tire size and crank length are irrelevant if all you want to do is compare gear inches, pick anything.

3. I use Sheldon Brown's gear calculator too, but I set it up for MPH at 90 RPM instead of gear inches. If you do that, you need to identify the tire size. I aim for a low speed of about 4.5 or 5. If I'm powering through a climb at 60 rpm, it brings my lowest speed down to 3 mph. If I'm going less than that, I either fall over or can go faster by walking.

For fat biking, I might go a little lower, just to make sure I can power through snow or smooth hills at low speed. Rocks & roots biking setup, 5 mph minimum.

4. I also find the conversion into speed at a given cadence on Sheldon Brown's gear calculator way more helpful than gear inches, because it converts it into something that means something to me. I know how fast I climb steep terrain, and I know how fast I go when I'm spinning on a fast downhill slope or a flat road so I can more easily determine what gearing I might need.

5. Looks like good information. I've wondered how much the size ( diameter) of the 2XL Snowshoe affects the size front and rear gears I would need.

6. I rode instead of doing calculations and the extra training made it not matter.

7. Originally Posted by tfinator
I rode instead of doing calculations and the extra training made it not matter.
There is something to be said for this, but there are times you're stuck behind a computer. Might as well do pointless hypothetical comparison of gearing from a 15 year old 26" full suspension to a brand new 1x 29er hardtail.

8. forty two

9. Originally Posted by Flucod
Thanks!
I hope you didn't take that seriously. Quick response from Hitchhikers Guide. There is no answer to "the best gear inches" question.

10. Originally Posted by watts888
There is something to be said for this, but there are times you're stuck behind a computer. Might as well do pointless hypothetical comparison of gearing from a 15 year old 26" full suspension to a brand new 1x 29er hardtail.
This is funny that you mention this because I think about it constantly. I know I used to climb some really hard hills in my early days of MTBing (plenty o' non-IMBA style trails then), so now I did all the calulations:
My old 26er (26 x 2.1) with 26-36-46 x 13-30 (low=22.5" to high=91.8") compared to my
29er (29x2.2) with 22-32-44 x 11-32 (low=19.9 to high=115.8) compared to the
fatbike (26 x 4.8) with 22-36 x 11-36 (low=17.6 to high=94.5).
I even had a 26er with 20-32-44 x 11-28 (low=18.5 to high=103.8).

I can't see any reason why I shouldn't be able to climb whatever I see, yet I can't always do it. So, IMHO, gear ratios are not all too important. Legs, lungs, and heart, OTOH, are very important.

Thanks to the OP for putting some logic in it.

-F

11. When setting up my rig for 1 x, I rode the 2 x and determined what I needed for the lowest gear. The two x 22/36 was just a bit too low and two x 22/36 was too high so I did my calcs this way.

22/36 = .611, which was too low for steepest hills

22/32 = .688, which was too high for steepest hills.

So I tried different chain ring sizes and came up with

26/42 = .619

26/40 = .65

I ended up putting a 26T chain ring on both bikes, but one with a 42T and the other with 2 40T cogs. This was determined by how steep a hill I was going to be climbing. There is a significant difference between .611 and .619, and even more so at .65. So numbers are great and a very good starting point. They should point you in the right direction. Trying different gears on your normal rides on hills or flats should be used in conjunction before relying strictly on numbers, and of course that will change if there's a significant difference in tire circumference.

Also remember that changing the chain ring up 2 teeth is going to result in a bigger difference than 2 teeth on a cog.

28/42 = .666 which is the sign of the beast on a steep hill.

28/40 = .7

12. Originally Posted by Fleas
This is funny that you mention this because I think about it constantly. I know I used to climb some really hard hills in my early days of MTBing (plenty o' non-IMBA style trails then), so now I did all the calulations:
My old 26er (26 x 2.1) with 26-36-46 x 13-30 (low=22.5" to high=91.8") compared to my
29er (29x2.2) with 22-32-44 x 11-32 (low=19.9 to high=115.8) compared to the
fatbike (26 x 4.8) with 22-36 x 11-36 (low=17.6 to high=94.5).
I even had a 26er with 20-32-44 x 11-28 (low=18.5 to high=103.8).

I can't see any reason why I shouldn't be able to climb whatever I see, yet I can't always do it. So, IMHO, gear ratios are not all too important. Legs, lungs, and heart, OTOH, are very important.

Thanks to the OP for putting some logic in it.

-F
I agree!!

Knowing your gear inches is very cool and may offer some insight when changing from one bike to another and you want the same feel. However from the start of the year to the end is a very different thing. First climb is generally a lot harder than the last of the year and obviously slower. Yes maybe it is cadence or gear but my bodies tells me what gear or cadence I can use.
Saying that, being aware of "why" may help to be more consistent and offer answers to WTF when trying another bike or gears.

Good info to keep in mind.

13. So I've got 30/11-42 (10spd) drivetrain on my AM whip... If I go 11-40... how much do I loose?

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#1 resolution... Ride it like I stole it!!

14. Originally Posted by targnik
So I've got 30/11-42 (10spd) drivetrain on my AM whip... If I go 11-40... how much do I loose?

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#1 resolution... Ride it like I stole it!!
It's like going from a unicycle with a 21" wheel to one with a 22" wheel. I suspect most of the time I would not be able to tell the difference (assuming I could ride a unicycle). YMMV

15. Originally Posted by targnik
So I've got 30/11-42 (10spd) drivetrain on my AM whip... If I go 11-40... how much do I loose?

-----------------------------------------------------------
#1 resolution... Ride it like I stole it!!
Grabbed values from Sheldon Brown gear calculator I compared 3 chainring sizes (all the way on the left) against a widerange cassette and a 42 cog added on at the end (shown on the top). The numbers underneath are the calculated speed you'd go if cranking at 80 rpm. Mashers go slower, spinners go higher. Changing the rear cassette from 42 to 40T with a 30T chainring will change the low end speed from 4.9 MPH to 5.2 MPH, a 6% decrease. Nothing crazy, pedal harder, go faster. Better yet, keep it in the 11T ring and go over 18 MPH.

MPH @ 80 RPM For 29 x 2.3 tires
- - -11 – 13 - 15 – 17 - 19 - 21 - 24 - 32 - 36 - 40 - 42
24 - 15.0-12.7-11.0-9.7-8.7-7.9-6.9-5.2-4.6-4.1-3.9
30- 18.8-15.9-13.8-12.2-10.9-9.8-8.6-6.5-5.7-5.2-4.9
36- 22.5-19.1-16.5-14.6-13.1-11.8-10.3-7.7-6.9-6.2-5.9

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